What happened to my salt?

[OCDP]

I went to go do a water change yesterday and my salt has gone all clumpy and chunky. When I mix the salt, it does not completely dissolve either??? I'm confused! I haven't done anything to the salt, the lid was on tightly sealed as always.. Has this happened to anyone else?? Sure is annoying, I think I'll have to buy another bucket as this stuff doesn't seem to want to completely dissolve.

What could have happened to the salt?

Thanks in advance guys,